This invention relates to an image processing apparatus, such as a copier, facsimile machine or printer, for processing image signals. More particularly, the invention relates to an image processing apparatus for pseudo-halftoning an input image signal to an image signal having fewer levels than the input levels.
Pseudo-halftone processing in an apparatus of this kind, e.g. a digital copier, generally makes use of the error diffusion method. The error diffusion method expresses halftones artificially by binarizing an 8-bit image signal using a predetermined threshold value and correcting a resultant binarization error by distributing the error to input signals indicative of the positions of pixels to be binarized subsequently, these pixels being adjacent a pixel of interest.
Conventionally, a digital copier of this type reads and records an image at a density of 400 dpi. Recently developed printers, however, have much higher resolution on the order of 600, 720 and 1200 dpi.
In order to raise the resolution of copiers which have a printing function as well, it is required that the image processing section which reads and processes the image be capable of operating at high speed. In addition, the memory of the two-dimensional processing section is required to have a very large storage capacity. This results in problems related to cost. For example, if the resolution of a system is to be raised from 400 to 600 dpi, then, in order to maintain the same copying speed, the processing speed of each image processing section must be increased by a factor of 2.25 owing to the need to read images. Further, in order for a spatial filter composed of a matrix of 5.times.5 pixels to obtain the same functionality, processing of a wide area of 7.times.7 pixels or more is required. This means that the storage capacity of a delay memory must also be increased 2.25 times, thereby raising cost.